Is AffinityMask Important??

Using the windows task manager to set affinity is very different from using the affinity mask in fsx.cfg.

With the fsx.cfg mask, fsx is told what cores it can use to run its threads, and it will specifically assign threads to the cores identified by the AM.  Whereas when the task manager assigns a process an AM, it tosses out the application's core assignments, and schedules all of the process threads to run on any core that is in the mask with the intent to load balance the threads over all of the AM cores.  This is why it appears that using the Windows TM AM gives a "better" uses of the cores.  But in reality this is really a bad idea for FSX since it carefully orchestrates the execution of its threads to avoid data clashes and race conditions.  When windows tosses this orderly procession of threads into a single big "free for all", those data clashes and race conditions will certainly happen, resulting in chaotic performance.

As far as fsx.cfg AM usage, I don't bother.  With my setup, I use cmd scripts to launch apps, such as AS16 and TrackIR, with AM masks that force them off core 0.  IMO this works very well, but of course YMMV.